1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser with an improved dielectric film formed on an end (front or rear) facet through which oscillating light passes.
2. Description of the Related Art
If a semiconductor laser is operated with the facets of the semiconductor crystal exposed, the semiconductor crystal facets will be oxidized and therefore the semiconductor laser will be gradually deteriorated. To prevent this problem, it is known, as shown in Japanese Unexamined Patent Publication No. 6(1994)-112679, for instance, that a transparent single-layer dielectric film, which consists of aluminum oxide (Al2O3), etc., is formed on an end facet of a semiconductor laser through which oscillating light passes.
In the case of such a dielectric film, the reflectance of the laser end facet varies with the thickness of the dielectric film. An example of the change characteristic of the reflectance is shown in FIG. 7. Hence, it becomes possible to set the reflectance of the laser end facet to a desired value by controlling the film thickness. Note that in FIG. 7 the axis of abscissas represents a ratio of optical length (refractive indexxc3x97thickness) to wavelength xcex, not the film thickness itself.
As is well known in prior art, if in semiconductor lasers the oscillating light emitted therefrom is reflected at the facets, etc., of other optical components and incident again as return light, the oscillation will become unstable, causing problems of noise, etc. To prevent problems with return light, setting the reflectance of the end facet of the semiconductor laser relatively low (in the order of 10 to 25%) is effective. For this purpose, a method of controlling the thickness of the above-mentioned dielectric film can be applied.
However, in the case where the above-mentioned single-layer dielectric film is formed on the laser end facet, it is considerably difficult to accurately set the reflectance of the laser end facet to a desired value, because, as is seen from FIG. 7, the reflectance of the end facet varies sharply with a change in the film thickness.
On the other hand, a curved line b of FIG. 6 shows a wavelength dispersion example of the reflectance of the end facet of a semiconductor laser with a single-layer Al2O3 film formed. As illustrated in the figure, the reflectance of the laser end facet varies sharply with a change in the wavelength. In many cases, semiconductor lasers in a certain oscillating wavelength band (e.g., 700 to 1100 nm, etc.) are fabricated by common processing. However, if the wavelength dispersion of the reflectance of the laser end facet is great as described above, semiconductor lasers that can be fabricated by common processing will be restricted to a considerably narrow oscillating wavelength band.
The present invention has been made in view of the above-mentioned circumstances. Accordingly, it is the primary object of the present invention to provide a semiconductor laser which is capable of minimizing a change in the reflectance of an end facet, through which oscillating light passes, with respect to changes in the refractive index and thickness of a dielectric film formed on the end facet, and accurately setting the reflectance of the end facet to a desired value.
To achieve this end and in accordance with an important aspect of the present invention, there is provided a semiconductor laser comprising: a first transparent dielectric film formed on at least either a front or rear facet through which oscillating light passes; a second transparent dielectric film formed on the first transparent dielectric film; and a third transparent dielectric film formed on the second transparent dielectric film; wherein the following relationships are satisfied:
0.09xcexxe2x89xa6n1d1xe2x89xa60.15xcex
0.20xcexxe2x89xa6n2d2xe2x89xa60.22xcex
0.225xcexxe2x89xa6n3d3xe2x89xa60.245xcex
1.58xe2x89xa6n1xe2x89xa61.64
2.0xe2x89xa6n2xe2x89xa62.4
1.44xe2x89xa6n3xe2x89xa61.46
wherexcex is the oscillating wavelength, n1, n2, and n3 are the refractive indices of the first, second, and third dielectric films with respect to the oscillating wavelength, and d1, d2, and d3 are the thicknesses of the first, second, and third dielectric films.
For example, aluminum oxide (Al2O3), titanium dioxide (TiO2), tantalum pentoxide (Ta2O5), and silicon dioxide (SiO2), which will be described later, are known as transparent dielectrics that are transparent to oscillating light emitted from a general semiconductor laser. In the present invention, the dielectric film materials are not limited to these. For instance, this invention is capable of employing a material containing aluminum (Al) and oxygen (o) and ranging from 1.58 to 1.64 in refractive index, a material containing titanium (Ti) and oxygen (o) and ranging from 2.2 to 2.4 in refractive index, a material containing titanium (Ta) and oxygen (o) and ranging from 2.0 to 2.2 in refractive index, and a material containing silicon (Si) and oxygen (o) and ranging from 1.44 to 1.46 in refractive index.
In accordance with another important aspect of the present invention, there is provided a semiconductor laser comprising: a first transparent dielectric film formed on at least either a front or rear facet through which oscillating light passes, the first transparent dielectric film consisting of aluminum oxide (Al2O3); a second transparent dielectric film formed on the first transparent dielectric film, the second transparent dielectric film consisting of titanium dioxide (TiO2) or tantalum pentoxide (Ta2O5); and a third transparent dielectric film formed on the second transparent dielectric film, the third transparent dielectric film consisting of silicon dioxide (SiO2); wherein the following relationships are satisfied:
0.09xcexxe2x89xa6n1d1xe2x89xa60.15xcex
0.20xcexxe2x89xa6n2d2xe2x89xa60.22xcex
0.225xcexxe2x89xa6n3d3xe2x89xa60.245xcex
wherexcex is the oscillating wavelength, n1, n2, and n3 are the refractive indices of the first, second, and third dielectric films with respect to the oscillating wavelength, and d1, d2, and d3 are the thicknesses of the first, second, and third dielectric films.
The refractive index n1 of Al2O3 ranges between 1.58 and 1.64, the refractive index n2 of TiO2 ranges between 2.0 and 2.4, the refractive index n2 of Ta2O5 ranges between 2.0 and 2.2, and the refractive index n3 of SiO2 ranges between 1.44 and 1.46.
Note that the aforementioned construction is particularly effective when applied to the case where the reflectance of the laser end facet is set relatively low (in the order of 10 to 25%) to eliminate the aforementioned problems with return light.
Reducing a change in the reflectance of the laser end facet with respect to changes in the refractive index and thickness of the dielectric film is equivalent to reducing reflectance distribution by wavelength, for the following reasons. From this fact it follows that if a dielectric film consisting of layers having a small change in the reflectance with respect to wavelength change is formed on the laser end facet, the object of the present invention is to be achieved.
Since there is a relationship of nd=xcex/4 between the refractive index n and thickness d of a single-layer dielectric film and the wavelength xcex of light at which the reflectance is the lowest, the following relationship is established:
(n+xcex4n)(d+xcex4d)=(xcex+xcex4xcex)/4
in which xcex4n is a variation in the refractive index n, xcex4d is a variation in the thickness d, and xcex4xcex is a change in the wavelength xcex. From this fact, if the layer construction of a dielectric film is formed so that the reflectances at wavelengths xcex and (xcex+xcex4xcex) are much the same, essentially the same reflectances can be obtained within the variation xcex4n in the refractive index and the variation xcex4d in the thickness. Thus, the object of the present invention can be achieved if a dielectric film, consisting of layers in which a change in reflectance distribution with respect to wavelength change is small, is formed on the laser end facet.
The above-mentioned three-layer dielectric film, constructed of the first dielectric film consisting of Al2O3, the second dielectric film consisting of TiO2 or Ta2O5, and the third dielectric film consisting of SiO2, which satisfies 0.09 xcexxe2x89xa6n1 d1xe2x89xa60.15xcex, 0.20xcexxe2x89xa6n2 d2xe2x89xa60.22xcex, and 0.225xcexxe2x89xa6n3 d3xe2x89xa60.245xcex, is clearly smaller in reflectance change with respect to wavelength change, compared with the conventional single-layer dielectric film (curved line b), as shown by a curved line a in FIG. 6.
The aforemenionted semiconductor laser, in which the materials of the first, second, and third dielectric films are not prescribed, is also capable of obtaining the same advantages as the aforementioned, because the refractive-index ranges of the first second, and third dielectric films are the same as the refractive-index range 1.58 to 1.64 of Al2O3, the refractive-index range 2.0 to 2.4 of TiO2 or Ta2O5, and the refractive-index range 1.44 to 1.46 of SiO2, respectively.
Therefore, the semiconductor laser of the present invention with such a three-layer dielectric film on the end facet is capable of minimizing a change in the reflectance of the end facet with respect to changes in the refractive index and thickness of the dielectric film, and accurately setting the reflectance of the end facet to a desired value, i.e., a value within the range of 10 to 25% required for eliminating problems associated with return light.
More specifically, as described later, even if the oscillating wavelength varies between xc2x1150 nm, reflectance within the range of 10 to 25% can be set to a desired value with an error range of xc2x11.5% (about 2% even in the case of leaving a margin). Thus, the semiconductor laser according to the present invention is capable of greatly enhancing its reproducibility, as it can be fabricated by a common end-facet coating process even when the oscillating wavelength varies within about 300 nm.
In addition, from the aforementioned relationship of (n+xcex4n)(d+xcex4d)=(xcex+xcex4xcex)/4, when the above-mentioned three-layer dielectric film is virtually taken to be a single-layer film, the refractive index and thickness of this film are taken to be nxe2x80x2 and dxe2x80x2, and, as described above, the oscillating wavelength varies between xc2x1150 nm, it can be safely stated that in a range where a value of (dxe2x80x2xcex4nxe2x80x2+nxe2x80x2xcex4dxe2x80x2+xcex4nxe2x80x2xcex4dxe2x80x2) becomes 37.5 nm (=150 nm/4), it is possible to suppress a change in the reflectance with respect to variations in the refractive index and thickness of the dielectric film to xc2x11.5% (about 2% even in the case of leaving a margin).